refactor(session): separate persistence behind SessionStore

Summary

• add a typed SessionStore contract
• move JSONL persistence into JsonlSessionStore
• keep caching, file-cap enforcement, and session forking in SessionManager

Why

This creates the storage boundary needed for a later SQLite backend without changing the current JSONL format or runtime behavior. SQLite schema and data migration are intentionally out of scope.

Tests

• pytest tests/agent tests/session tests/test_nanobot_facade.py tests/webui/test_session_list_index.py -q (1569 passed, 2 skipped)
• ruff check nanobot/
• targeted BasedPyright check (0 errors)